Electro-magnetic waves, in the frequency range located between infrared and microwave, are one of the latest developed areas of spectrum. This band is often referred to as the terahertz (THz) band. These waves are transparent for most non-polar dielectrics, such as plastic, paper, stone, wood, oil, smoke, et. al. This makes THz wave imaging an ideal tool as a complement to x-ray and ultrasound imaging in security inspection and quality control applications. Additionally, THz wave imaging provides spectroscopic information on the target, and it can be used to identify the target. THz wave imaging is safe to both the sample and the operator as THz photons have very low energy (meV), which will not ionize molecules.
THz wave imaging technology such as described in U.S. Pat. No. 5,623,145 and U.S. Pat. No. 5,710,430 to M. Nuss, demonstrated the capability of seeing through plastic and mapping metal electrodes underneath. Most THz wave imaging apparatuses developed since then use a raster scanning mode to image the target. THz radiation emitted from the source is focused on the target imaged, and is recorded using a point detector after interacting with the target through transmission or reflection. The target is imaged via scanning the target crossing the THz wave focal spot in an X-Y plane or alternatively scanning the imager in opposite directions. Raster scanning fully utilizes THz waves generated from the source and results in a high measurement dynamic range, which is especially important in the THz regime where lack of intense sources and sensitive detectors pose a problem.
Raster scanning an image requires linearly scanning either the target or the imager within the entire image area, which is not only time consuming but also inconvenient. 2D focal plane imaging, which uses a 2D extended detector (such as an electro-optical (EO) crystal) or a detector array (such as a microbolometer array) instead of a point detector to record the distribution of the THz field at the image plane, was developed to improve imaging speed. An example is reported in “Two-dimensional electro-optic imaging of THz beams,” Appl, Phys. Lett., 69, 1026-1028 (1996) by Q. Wu et. al., where a single crystal EO sensor was employed as the extended THz wave sensor. An intense THz beam, which was extended illuminating the entire target, was generated using a femtosecond (fs) laser amplifier through an optical rectification or an optical switching process. The THz wave image of the target was created using an imaging lens and the EO crystal was placed at the image plane. An extended probing beam was used to read out the THz field distribution on the EO crystal, which was the THz wave image of the target.
Detector arrays, and 2D focal plane imaging methods, have also been developed for cw THz radiation under certain circumstances. One example was reported in “Real-time, continuous-wave terahertz imaging by use of a microbolometer focal-plane array,” Opt. Lett. 30, 2563-2565 (2005), by A. W. Lee et. al., where a microbolometer array, which was designed for middle infrared radiation, was demonstrated to have the capability to record 2D images at a high THz frequency (2.52 THz). Other examples include U.S. Pat. No. 6,242,740 to A. Luukanen et. al., and U.S. Pat. No. 6,943,742 to S. Holly.
Prior art 2D focal plane THz wave imaging systems, however are only available under limited conditions. Especially, these detector arrays are only sensitive at limited frequency ranges. They also require a relatively more intense THz source than point detectors due to the low sensitivity of detector arrays and dilution of the THz radiation on multi pixels at one time. Alternative methods were therefore developed to improve imaging speed without using an extended detector or detector array. Prior art U.S. Pat. No. 6,909,094 and U.S. Pat. No. 6,909,095, to P. Tran et. al., use a distributed waveguide technique to receive THz wave image at the image plane and using a point detector to receive signals from all pixels sequentially. “Terahertz wave reciprocal imaging,” Appl. Phys. Lett. 88, 151107 (2006), by J. Xu et. al, reported using a source array rather than a detector array to present a focal plane image in a reciprocal way. These arts avoid using 2D detector arrays, which are not available or not ideal for THz waves, however there are still other challenges when using a waveguide array or a source array.
U.S. Pat. No. 6,815,683 to J. Federici adapted an interferometric imaging technique from the microwave regime to the THz regime. Using only a few detectors and without using an imaging lens, this technique promises to present large scale THz wave imaging without moving imager or target. On the other had, this art utilizes THz radiation in an inefficient way, as no collection optics is allowed in this technology.
Compared to arts using distributed detection or emission components, the raster scanning method is still the predominantly used imaging method for THz radiation because it provides higher measurement dynamic range, gives better image quality and is available for all kinds of THz wave sources and detectors.
Efforts have also been taken to improve the speed of THz wave raster scanning image. An example was described in WO 02/057750 by B. Cole et. al., where a THz wave was guided into an imaging head, which contains only a limited number of optics (and THz optics). It was the imaging head rather than the entire imager being scanned in then imaging process. The speed of imaging was improved because less mass was scanned.
The detectivity of THz waves currently available is not sufficient to utilize these waves for an imaging device in a comparable fashion to other well developed areas of wavelength ranges. Furthermore, the lack of a strong source to overcome the detectivity problem is also challenging. The immature source technology also leads to difficulties in manipulating the wave to be accommodated into other techniques. As a result, detection of scattered light out of a target to be imaged is not a practical method, due to the lack of strong source. Equally, realization of a modulation technology to enhance the detectivity of a weak source is not feasible due to immature technology surrounding the source. Speed of imaging is another important factor in considering an effective imaging system.
Currently, no prior art is known for an imaging apparatus in the range of extended THz frequencies, e.g., 1 GHz˜100 THz, disclosing a concept for realization of practically usable sensitivity and speed. This disclosure addresses these two major points with an exemplary embodiment focusing on two dimensional raster scanning of an image by scanning the THz beam across each spot of the target using a wave in the frequency range of 10 GHz˜3 THz.